10-speed cassette on 8-speed hub?
 

I have a new Ultegra 10-speed cassette and an older 8-speed Dura-Ace hub. With the cassette on the hub, there is significant play in the cogs such that there is not a tight fit. Can this problem be safely fixed with spacers?

Yes.

edit: Well technically we didn't have the same deal - I was going between 9 and 10 speed stuff and fixed a fair amount of play with a spacer, so I'd imagine you could too. One problem I ran in to - my Mavic wheels needed a spacer but my Ultegras didn't, so I had to grind down the spacer and put it on the outside of the smallest cog in order to not have to re-adjust my derailleur every time I switched wheels.

The 10sp cassette needs a 1mm spacer when put on a 9 or 8sp freehub. It should come with the cassette, or get some from the LBS.

Mavic hubs also require a Mavic spacer for both 9 and 10sp cassettes.

An 11 tooth cassette can not be used only a 12. So if you are like me and don't need an 11 no problem.

The cassette does have an 11 tooth cog; it's an 11-23. This will prevent it from working (with spacers) on the 8-speed hub?

The cassette body will not accept an 11. Nothing to do with spacers. The cassette bodies that DO accept an 11 are necked down a little at the end. The 8 speed didn't have this and the 9 speed did if I remember right.

I see. Thank you for your advice.